Solar cells relate to technologies for the collection and conversion of solar energy to electrical energy. The efficiency of solar cells may be measured in terms of “external quantum efficiency” or “EQE.” EQE may be defined as the ratio of a number of charge carriers collected by a solar cell to the number of photons having a particular energy (e.g., wavelength) shining on the solar cell. Thus, EQE may be used to measure an efficiency of energy collection associated with a particular solar cell.
Solar cells may be constructed from a variety of materials (e.g., germanium, to name but one), and, depending upon the material or materials comprising a particular solar cell, the cell may best absorb energy in a particular part of the solar spectrum. In general, a particular solar cell (constructed from one or more materials) will operate most efficiently (i.e., the solar cell will have an EQE closest to 1 or 100%) in a particular frequency range.
In view of the relatively narrow absorption range associated with typical solar cells, and in order to collect a larger proportion of energy in the solar spectrum, many modern solar collection devices incorporate a variety of monolithically layered solar collecting materials or “subcells” where each subcell is best suited to absorption of photons in a particular frequency range. The boundary or interface between one subcell and another subcell may be referred to herein as an “interface,” and these devices may be referred to as multi-junction solar cells (“MJSCs” for short). Where MJSCs are used, a top subcell may be constructed to absorb light of a shorter wavelength, while a bottom subcell may be constructed to absorb light of a longer wavelength. Intermediate subcells may absorb light in the spectrum between light absorbed by the top and bottom subcells.
As the number of interfaces used in the construction of MJSCs increases, it is increasingly important that the subcells are current matched (e.g., because the current through an MJSC may be limited by the minimum current flowing through a subcell in a series coupled set of subcells). To achieve current matched subcells, EQE calculations are often used in the design and development of MJSCs. Because EQE is a measure of the photocurrent generated by each subcell under a certain portion of the solar spectrum, the EQE of each subcell in an MJSC may be measured during a design phase to ensure that the subcells are properly current matched.
The accurate measurement of EQE is important to the design and development of MJSCs. For monolithically integrated MJSCs, subcells may be series connected and coupled both electrically and optically. The low shunt resistance of a subcell and the luminescence coupling between subcells may cause EQE measurement artifacts that obscure the subcell intrinsic properties.
Simple application of a particular portion of the solar spectrum to an MJSC may well excite some current in a subcell that is not precisely tuned to that portion of the spectrum, but which is nevertheless at least partially responsive to that portion of the spectrum. Moreover, if subcells which are not under test are not light biased during testing to achieve substantial output currents, these subcells may current limit the subcell under test (which may interfere with collection of an EQE value for the test subcell). Thus, it is typically necessary to DC light bias and/or DC voltage bias subcells not under test to make the subcell under test the current limiting subcell.
Due to the effects described above, and in spite of the precautions taken against their occurrence, measurement artifacts (e.g., leakage currents) are often observed in the outputs of tested subcells (e.g., often measurement artifacts are observed for subcells with low shunt resistances and/or strong luminescence coupling effects). These measurement artifacts are characterized, for example, by the occurrence of erroneous output responses outside the wavelength or frequency range to which a subcell is tuned, and/or by the occurrence of low output responses within a subcell's tuned wavelength range. These measurement artifacts may be decreased by applying a DC light and/or voltage bias. However, the difference between the measured apparent EQE and the true EQE may still be very substantial. Thus, improved techniques for characterizing the EQE of subcells under test are desirable.